1. Field of the Invention
The present invention relates generally to electronic communication. More specifically, the present invention relates to the in-band generation of low-frequency periodic signaling.
2. Description of the Related Art
Universal Serial Bus (USB) protocol is a standard communications protocol for sending signals between devices and host controllers. USB has become the standard mechanism to communicate between computer devices, and is commonly used to link peripherals, such as keyboards, mice, external disk drives, etc. with personal computers. It is also quite commonly used to link various self-sufficient devices, such as cellular phones, portable media devices, etc. with personal computers.
USB 3.0, also known as SuperSpeed USB, is the latest iteration of the USB standard. It incorporates a “SuperSpeed” bus to achieve communication speeds of up to 5.0 Gb/s. Part of the USB 3.0 protocol describes the use of Low-Frequency Periodic Signaling (LFPS). Specifically, a high-frequency oscillator is used to generate high-frequency communications for the main data communication (e.g., content) that is being transferred between devices. However, a separate low-frequency oscillator is used for low-frequency sideband communication between two ports of a link in a low power state for various auxiliary functions, such as signal initialization and power management. It is also used when a link is under training, or when a downstream port issues a Warm Reset command to reset a link.
LFPS is commonly used when the link is in a low power mode, and thus a low power low-frequency oscillator is used for such communications. The low-frequency oscillator's clock is neither frequency nor phase synchronous to the high-frequency oscillator's clock, which is typically a PLL generated clock used for high-speed data generation. Thus, while the low-frequency oscillator uses less power than the high-frequency oscillator, it is in a different clock domain and requires separate management, which adds complexity and cost to any circuits designed for the transmission and receipt of the LFPS signals.
What is needed is a solution that does not suffer from these issues.